The need to provide duplicated data transmission paths in telecommunication switching systems and computers is well known. One such system which utilizes duplicated data transmission paths to interconnect switching modules to a central switch is the AT&T 5ESS.RTM. switching system which is described in the AT&T Technical Journal, Volume 64, Jul.-Aug. 1985, No. 6, Part #2. In the 5ESS switching system, the switching modules are called time-slot interchangers (TSIs) and the central network is referred to as time-multiplexed switch (TMS). Each TSI is interconnected to the TMS by four optical links. One pair of optical links is used for transmission from the TSI to the TMS and the other pair of optical links is used for transmission from the TMS to the TSI. For either pair, one link of the pair is the active link, and the other link of the pair is the standby link. If the active link fails, the units use the standby link. In addition, during normal operation the TSI and the TMS alternate between which optical link is the standby and which link is the active. This control is performed by sending the control messages within data (in-band signaling) that is being transmitted over the active link. In-band signaling control suffers difficulties in synchronizing the transfer between the active and the standby. Further, if the active link fails, the control signal fails with it and switching to the standby link may be delayed since the loss of the control signal can only be checked for at predetermined intervals. This type of in-band signaling is also used in digital cross-connect systems (DCS).
In addition, it is known in the prior art to utilize separate transmission links for transmitting the control signals designating which of the duplicated links is to be active. This suffers from the problem of being expensive. For example, in the case of the 5ESS switching system, this would require the addition of four more optical links. Not only is the cost of optical links very expensive but often there is a large space penalty when the amount of cabling becomes excessive. This is particularly true in DCS systems which require a large number of links, and the ability to save four links on each duplicated bilateral path has an immense effect on the cost of the system plus the amount of physical space that is utilized by the system.
There exists a need in the prior art for providing control signals on each transmission path between two units to indicate which of the duplicated transmission paths is active by providing this within the transmission path without utilizing inband signaling.